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#include "precomp.hpp"

namespace cv { namespace hal {

/****************************************************************************************\
*                     LU & Cholesky implementation for small matrices                    *
\****************************************************************************************/

template<typename _Tp> static inline int
LUImpl(_Tp* A, size_t astep, int m, _Tp* b, size_t bstep, int n, _Tp eps)
{
    int i, j, k, p = 1;
    astep /= sizeof(A[0]);
    bstep /= sizeof(b[0]);

    for( i = 0; i < m; i++ )
    {
        k = i;

        for( j = i+1; j < m; j++ )
            if( std::abs(A[j*astep + i]) > std::abs(A[k*astep + i]) )
                k = j;

        if( std::abs(A[k*astep + i]) < eps )
            return 0;

        if( k != i )
        {
            for( j = i; j < m; j++ )
                std::swap(A[i*astep + j], A[k*astep + j]);
            if( b )
                for( j = 0; j < n; j++ )
                    std::swap(b[i*bstep + j], b[k*bstep + j]);
            p = -p;
        }

        _Tp d = -1/A[i*astep + i];

        for( j = i+1; j < m; j++ )
        {
            _Tp alpha = A[j*astep + i]*d;

            for( k = i+1; k < m; k++ )
                A[j*astep + k] += alpha*A[i*astep + k];

            if( b )
                for( k = 0; k < n; k++ )
                    b[j*bstep + k] += alpha*b[i*bstep + k];
        }

        A[i*astep + i] = -d;
    }

    if( b )
    {
        for( i = m-1; i >= 0; i-- )
            for( j = 0; j < n; j++ )
            {
                _Tp s = b[i*bstep + j];
                for( k = i+1; k < m; k++ )
                    s -= A[i*astep + k]*b[k*bstep + j];
                b[i*bstep + j] = s*A[i*astep + i];
            }
    }

    return p;
}


int LU(float* A, size_t astep, int m, float* b, size_t bstep, int n)
{
    return LUImpl(A, astep, m, b, bstep, n, FLT_EPSILON*10);
}


int LU(double* A, size_t astep, int m, double* b, size_t bstep, int n)
{
    return LUImpl(A, astep, m, b, bstep, n, DBL_EPSILON*100);
}


template<typename _Tp> static inline bool
CholImpl(_Tp* A, size_t astep, int m, _Tp* b, size_t bstep, int n)
{
    _Tp* L = A;
    int i, j, k;
    double s;
    astep /= sizeof(A[0]);
    bstep /= sizeof(b[0]);

    for( i = 0; i < m; i++ )
    {
        for( j = 0; j < i; j++ )
        {
            s = A[i*astep + j];
            for( k = 0; k < j; k++ )
                s -= L[i*astep + k]*L[j*astep + k];
            L[i*astep + j] = (_Tp)(s*L[j*astep + j]);
        }
        s = A[i*astep + i];
        for( k = 0; k < j; k++ )
        {
            double t = L[i*astep + k];
            s -= t*t;
        }
        if( s < std::numeric_limits<_Tp>::epsilon() )
            return false;
        L[i*astep + i] = (_Tp)(1./std::sqrt(s));
    }

    if( !b )
        return true;

    // LLt x = b
    // 1: L y = b
    // 2. Lt x = y

    /*
     [ L00             ]  y0   b0
     [ L10 L11         ]  y1 = b1
     [ L20 L21 L22     ]  y2   b2
     [ L30 L31 L32 L33 ]  y3   b3

     [ L00 L10 L20 L30 ]  x0   y0
     [     L11 L21 L31 ]  x1 = y1
     [         L22 L32 ]  x2   y2
     [             L33 ]  x3   y3
     */

    for( i = 0; i < m; i++ )
    {
        for( j = 0; j < n; j++ )
        {
            s = b[i*bstep + j];
            for( k = 0; k < i; k++ )
                s -= L[i*astep + k]*b[k*bstep + j];
            b[i*bstep + j] = (_Tp)(s*L[i*astep + i]);
        }
    }

    for( i = m-1; i >= 0; i-- )
    {
        for( j = 0; j < n; j++ )
        {
            s = b[i*bstep + j];
            for( k = m-1; k > i; k-- )
                s -= L[k*astep + i]*b[k*bstep + j];
            b[i*bstep + j] = (_Tp)(s*L[i*astep + i]);
        }
    }

    return true;
}


bool Cholesky(float* A, size_t astep, int m, float* b, size_t bstep, int n)
{
    return CholImpl(A, astep, m, b, bstep, n);
}

bool Cholesky(double* A, size_t astep, int m, double* b, size_t bstep, int n)
{
    return CholImpl(A, astep, m, b, bstep, n);
}

}}